I am curious what I should fill the oil tank with these days? I was planning on just using that new synth stuff they sell at the hardware store for yard tools?

1980 MX80

Cool bike! I still have a 1980 YZ 50, was my son's first bike, he's 22 years old now, over 6 ft. tall. For your oil tank, refer to the premix thread, there are specific oils made for injection. Use an oil made for injection, such as the Yamalube 2-S, not the 2-R (again, see premix thread). Buy it from a Yamaha dealer, not Home Repo.